The invention is related to sodium transition metal cathode materials for a rechargeable sodium battery. The transition metal is composed of manganese and a minority of various other metals.
There are worldwide efforts to limit consumption of fossile fuels by introducing “green cars” like electric vehicles. Currently the trend is to use Li rechargeable batteries for these cars. However, this approach is disputed because it might be environmentally difficult to produce sufficient lithium precursors in a sustainable way.
Lithium is abundant; the content of Li only in seawater is estimated to be 230 billion tons. However, lithium is generally occurring in low concentrations which limits the practically available recourses to about 13 million tons according, to a recent survey. These resources are insufficient for a world full of “green cars”. Besides efforts to expand the availability of lithium, for example by extraction from seawater, Li free rechargeable batteries, such as sodium rechargeable batteries, receive renewed interest.
Besides Li scarcity other potential issues are not fully solved: LiCoO2, NMC (LiMO2 where M=LiaM′1-a, M′=Ni1-x-yMnxCoy and LNCO (LiMO2 with M=Ni1-x-yCoxAly) based cathode materials have the highest energy density, but they contain scarce cobalt and relatively expensive nickel. Cobalt and Nickel free cathodes like spinel (LiMn2O4) or olivine (LiFePO4) have the disadvantage of a lower gravimetric energy density. Other potentially cheap and high energy cathode materials are far from commercialization because major issues (like cycle stability, gas generation, compatibility with current electrolytes) are not solved yet.
Examples of these are:
1) Li and Mn based cathode materials (within this patent application we refer to them as “HLM”) being a solid state solution of LiMO2 with Li2MnO3 and
2) high voltage spinel materials (LiNi1/2Mn3/2O4)
Li batteries for green cars have very tough criteria to achieve a sufficient calendar life (or cycle stability) and safety. Consumers expect that the battery still works well after 10 years of use; consumers also expect a high safety. Large Li batteries always have the potential to catch fire or—in the worst cases—to explode. Cycle stability and safety have been improved, but further improvements are required.
In many aspects sodium behaves electrochemical similar to lithium. The theoretical possibility of sodium ion batteries is known since the late 70-ties. The successful reversible intercalation and de-intercalation of sodium ions in a cathode material (NaxCoO2) has been described already 1981 by Delmas, who showed data of half cells (Na-cathode and sodium metal anode). However, similar to Lithium batteries, sodium metal cannot be used as anode material in commercial batteries due to dendrite formation, so an anode material adapted to sodium ion batteries is required. In 1999 a group at Dalhousie University demonstrated hard carbon as Na intercalation anode material. Since that time the feasibility of Na batteries is in principle accepted, however, in the shade of the ever-increasing Li rechargeable battery world only relatively few efforts were assigned to develop practical sodium intercalation batteries.
Sodium ion batteries are thus in their early stages of technological development. It is already clear now that layered sodium ion cathodes can be cheap because they need less cobalt or can even be cobalt free. The possibility to avoid cobalt is related to basic crystal-physical properties. Sodium bronzes are perfectly layered without cobalt, whereas in LiMO2Co is important to stabilize the layered crystal structure, and to prevent cation rearrangement (migration of transition metal to lithium sites) during cycling.
In “P2-type Nax[Fe1/2Mn1/2]O2 made from earth-abundant elements for rechargeable Na batteries” by Yabuuchi et al. (in Nature Materials 11 (2012), 512-517), a cathode material NaxFe1/2Mn1/2O2 has been disclosed. This cathode material does not contain scarce or toxic elements, and it shows high reversible capacity in Na ion batteries. The cathode material is a true intercalation material. The Fe—Mn-oxide host structure remains intact during sodium de-intercalation and re-insertion. Sodium ion batteries containing high capacity sodium cathodes like NaxFe1/2Mn1/2O2 and anodes like hard carbon increasingly will be able to challenge the dominance of Li ion batteries, especially for green cars. However, Na—Fe—Mn-oxide cathodes are not sufficiently air stable and are difficult to prepare. Another class of known sodium intercalation cathode materials are phosphates like NaMn0.5Fe0.5PO4, or fluorite-phosphates like NaVPO4F or Na2FePO4F. However, currently no commercial cathode materials for sodium ion batteries exist. All currently proposed materials show problems. For example vanadium oxide based cathodes are toxic, and phosphates or fluorophosphates have a low gravimetric capacity. Na—Co-oxide cathode materials are expensive since they contain scarce cobalt.
Some prior work about manganese based Na containing cathode materials exists: patent applications US 2010/0248040 A1 and US 2011/0200879 A1 by Sanyo teach to apply cathodes which contain sodium as well as lithium for Li ion batteries (not sodium ion) batteries. US 2010/0248040 A1 describes NaaLibMxO2 where M is selected from Mn, Fe, Co, Ni and where the diffraction pattern has a peak at 15.9-16.9 degree. The cathodes do not contain magnesium, and the cathodes are used in Li ion batteries. A special charge plateau due to high Mn valence state is not observed.
In “Studies of the layered manganese bronzes, Na2/3[Mn1-xMx]O2 with M=Co, Ni, Li, and Li2/3[Mn1-xMx]O2 prepared by ion-exchange” by Paulsen and Dahn (in Solid State Ionics 126 (1999) p. 3-24), and in “Layered T2-, O6-, O2-, and P2-Type A2/3[M′2+1/3M4+2/3]O2 Bronzes, A=Li, Na; M″=Ni, Mg; M=Mn, Ti″ by Paulsen et al. (in Chem. Mater. 2000, 12, 2257-2267) the sodium bronzes are not used as sodium cathode materials but are precursors for preparing Li-transition metal oxides by ion-exchange.
It remains an open question if sodium rechargeable battery technology will or will not allow to achieve a higher calendar live (cycle stability) ion cathode, a better safety, and this at a reduced cost or with an improved performance, by allowing a different design when compared to lithium ion battery technology. It is however clear that rechargeable sodium battery technology has the potential to replace lithium technology if cheap sodium cathode materials with high capacity allow to achieve good calendar life and high safety at low cost. The latter problem is solved by the present invention.